Well, if you had enough fans then it would be good to put them in all those aforementioned positions.

But if you could only choose ONE place to install a case fan, then in my opinion one should definitely choose the rear exhaust fan position. In an ATX tower case, this is located right under the power supply, and is in close proximity to the CPU's HSF, providing extra cooling for the processor.

This fan mount is present in most cases, and would also provide cooling to the system as a whole because of its relatively elevated position in relation to the rest of the case (warm air rises, convection).

But if you had enough fans:
- the front fan would be near the bottom of the front panel, blowing in
- the side fan would be on the side panel, blowing in
- the rear fan would be near the top of the back of the case, blowing out

I've found that the side fan blowing in can cause issues on the CPU fan blowing in.
Just through experience

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I don't quite understand. A stock Intel CPU heatsink fan blows into the heatsink, and a side fan shouldn't cause issues there, if there's space.

As for the original question, quite definitely the rear exhaust fan. I've wrecked a computer in my earlier years of messing around with computers by installing the fans in the front, sucking air in, and no fans at the rear, thinking that the PSU already had a fan.

Turned out, it wasn't sufficient at all, and in a year, the PSU burnt out, and took the motherboard with it. Not to mention that it kept overheating.

Oh well, learnt from it, and hopefully you won't consider any other fan placements, for your solo fan XD

Btw, it generally is enough to have a solo exhaust fan, although it is recommended that you should have a front intake fan as well. Of all the fans, the side panel fan is the least useful in terms of general PC usage. If you're after absolute lowest CPU temps, its a different story altogether.

The top 200mm fan on the Antec Nine Hundred is a clever way to exhaust a lot more air while creating less noise. Again, since warm air rises, it makes perfect sense for Antec to put a large fan on the roof of that particular case.

Although the 200mm top fan is a great idea, it still does not replace the back exhaust fan. Hot air rises, that much we can agree on, but it does not rise at a rate fast enough. Given the back fan would be much closer to the CPU, it would perform much better in terms of exhausting hot air.

HOWEVER, If you got one of this, these, or similar heatsink design, you can get away without an exhaust fan, since the fan on these heatsinks will blow hot air straight out. HOWEVER (again), I wouldn't recommend skimping on that rear exhaust fan.

Wait, looking at your first post, I'm assuming that you DON'T have a fan anywhere at all at the moment.

If that is the case, I strongly suggest the BACK of the case, not the front or side.

That is the most important place to put a fan, as having an intake fan blowing onto your CPU alone will cause hot air to stagnate on other parts of the computer case, which may be more disastrous. A slightly higher CPU temp will not cause any problems, however a burnt out PSU (which will happen, even though your PSU has a fan itself) will be disastrous.

Also, I'd like to point out to you that ALL cases come with back and front fan placements, but only some, and even then, only the newer cases will come with side fan placements. This is due to the fact that there is already enough cooling for the CPU with a single rear fan, and an extra side fan will further lower the temps, but will not have a huge impact on normal computing (overclockers excepted of course).

Then again, it is your computer, and if you still feel that a single side fan is adequate for your needs, feel free to do as your please

I believe in the fact that you can never overcool a PC but that pic takes it a bit too far.
As for the fan, the first priority would be to place it so it sucks hot air out of the case and throws it out the back. Next would probably be some fans in the front and lastly, a fan or two on the side panel.